Marine parasites such as trematodes are known to modify the phenotype of their hosts by interfering with growth rates, behaviour, reproduction and survival. Such effects are generally considered to be density-dependent, i.e. the greater the infection level in the host, the greater the detrimental impact on host fitness. However, the mechanisms determining infection levels in marine hosts are still poorly understood. Here, we investigated the effect of cercarial dose and exposure frequency (single vs. trickle infections) of a marine trematode parasite (Himasthla elongata) on infection levels in its second intermediate host, the blue mussel Mytilus edulis, an abundant and widely distributed bivalve in European coastal waters. For more detail see publication.